The aim of our study was to evaluate the hormonal profile in a group of 31 subjects who underwent recombinant interferon-α therapy for chronic active hepatitis C. Hormonal determinations were performed before treatment began and at the end of the 3rd and 6th months of therapy. Free-T<sub>4</sub> concentrations, though remaining in the normal range, showed a significant reduction (p < 0.05) after 3 and 6 months of therapy compared with pretreatment levels. A lesser decrease in free-T<sub>3</sub> levels was also seen. TSH basal values did not show any variation, while an increased secretory response to TRH stimulation was observed at the end of the 6th month. Thyroglobulin and calcitonin levels remained normal, while an increase in antithyroglobulin and antithyreoperoxidase antibody levels was observed in 4 patients (12.9%). No modifications in the other pituitary hormones or in adrenal and sex steroid concentrations were noticed. A significant increase in IGF-I concentrations (p < 0.05) was observed during treatment, and an inverse correlation was seen between IGF-I and alanine aminotransferase levels (p < 0.01). This study supports the view that interferon treatment can influence thyroid function. The increase in IGF-I concentration observed during therapy may reflect an improvement in patients with hepatic disease, but a direct stimulatory effect of interferon on IGF-I secretion cannot be excluded.